The present invention relates to multiple antenna wireless access points that multiplex signals by spatial division. In particular, it relates to grouping clients of the access point for spatial division multiplexing.
Wireless LAN (WLAN) access points are a popular product category. As the number of clients that desire service from an access point increases, one strategy for increasing the number of clients served is spatial division multiplexing (SDMA). Spatial diversity involves the use of multiple antennas sufficiently well-separated at the receiver and/or the transmitter that the individual transmission paths experience effectively independent fading. Juha Heiskala and John Terry, OFDM Wireless LANs: A theoretical and Practical Guide 2002 p. 131. Some theoretical investigation has been directed to the effectiveness of spatial multiplexing, such as Helmut Bölcskei et al., “On the Capacity of OFDM-Based Spatial Multiplexing Systems”, IEEE Transactions on Comm., Vol. 50, No. 2 February 2002; Helmut Bölcskei et al., “Space-Frequency Coded MIMO-OFDM with Variable Multiplexing-Diversity Tradeoff”, IEEE 2003; and Arogyaswami J. Paulraj and Boon Chong Ng, “Space-Time Modems for Wireless Personal Communications”, IEEE Personal Communications• February 1998. A survey of smart antenna literature has been prepared by Seshaiah Ponnekanti, “An Overview Of Smart Antenna Technology For Heterogeneous Networks”, IEEE Communications Surveys, Vol. 2 no. 4, http://www.comsoc.org/pubs/surveys Fourth Quarter 1999, pp. 14 et seq.
Work on use of multiple antennas to improve wireless communications has included studies of selecting antenna subsets, utilizing a partial MIMO instead of a full MIMO. Dhananjay A. Gore and Arogyaswami J. Paulraj, “MIMO Antenna Subset Selection With Space-Time Coding”, IEEE Transactions On Signal Processing, Vol. 50, No. 10, October 2002; and A. Gorokhov, D. Gore and A. Paulraj, “Performance Bounds for Antenna Selection in MIMO Systems”, IEEE 2003. Use of a subset, instead of the whole set of available antennas, sacrifices antenna diversity for reduced computation and signal translation.
In general, an access point that processes signals from “m” antennas can simultaneously service m−1 clients by spatial division of signals. Theoretically, solving a linear system of differences among reception of signals at m antennas can resolve m−1 sources. The problem is more difficult than just resolving the m−1 sources, because an additional number of interference sources or interferers need to be rejected and noise needs to be filtered out.
Given the opportunity to extend the functionality of radio wireless access points by using multiple antennas and spatial division multiplexing, an opportunity arises for methods and devices that improve the performance of SDMA had a modest cost.